1. Field of the Invention
This invention relates to a control device for a vibration wave motor in which a periodic voltage is applied to an electro-mechanical energy conversion element such as an electrostrictive element to form a travelling vibration wave in the surface of a vibration member and a moving member is driven by said vibration wave.
2. Related Background Art
In the vibration wave motor of the above-described type, it is known to adjust the frequency of said periodic voltage in adjusting the rotational speed of the motor.
That is, the motor of the above-described type has a characteristic that as the frequency of the periodic voltage applied to the electro-mechanical energy conversion element is made lower, the rotational speed of the motor becomes higher, and the frequency has been made variable for the speed adjustment to thereby accomplish speed control.
As described above, in the motor of the described type, as the drive frequency thereof is made lower, the speed thereof becomes higher, but such motor has a characteristic that the rotational speed is abruptly reduced when the frequency is reduced beyond the resonance frequency of the motor.
Accordingly, in controlling the speed of the motor, it is necessary to prevent said drive frequency from becoming lower than the resonance frequency of the motor, and as a method therefor, it is conceivable to store the resonance frequency of the motor and control the drive frequency so that the drive frequency does not become less than the stored frequency.
However, the resonance frequency in the motor of the described type is varied by environmental influences such as temperature, etc. and therefore, even if the resonance frequency is stored as a fixed value and said control is effected, the actual resonance frequency differs from the stored value and said inconvenience cannot be prevented.
Also, when the motor is to be set to a desired speed, a frequency corresponding to the desired speed may be selected and applied to said electro-mechanical energy conversion element, but if the frequency corresponding to said desired speed is directly selected at the starting of the motor, the motor abruptly starts to rotate and cannot be smoothly rotated.
To eliminate such problem, the drive frequency may be gradually reduced from a high frequency when the motor is started, and may be shifted to the frequency corresponding to the desired speed.
If the high frequency at the starting of the motor is set to a frequency at which the motor actually starts to rotate and the frequency is gradually reduced from this set frequency, the motor can be driven to the desired speed smoothly within a short time, whereas the characteristic of said motor is not constant but is affected by changes in the environmental conditions such as temperature, etc. and if the frequency at the starting of the motor is fixed, any change in temperature or the like cannot be coped with, and this leads to inconveniences such as said abrupt rotation and a long time required until the starting of the motor.